FIELD OF THE INVENTION
The invention relates to a method and device for controlling sheet feed to a sheet-processing machine, especially to a printing machine.
In order to avoid damage to a printing machine, it has become known heretofore to provide sensors or detectors which respond to an incorrect feed. An incorrect feed exists when more than the contemplated number of sheets is being fed, when the position, the thickness or the material of the sheets is not as desired, or when the sheets are damaged, have holes formed therein or are dog-eared, i.e., have folded-over corners. In machines which process a great number of sheets per unit time, the sheets, which have been singly separated from a sheet pile, are conveyed onto a feed table in imbricated or overlapped form and, for processing, are individually brought to alignment stops. In this type of sheet feed, the requirements imposed upon the detectors are particularly high. Improvements have become known wherein, in order to increase security and reliability, several detectors are provided in the conveying path of the sheets. In order to be sensitive to various sheet materials, detectors may be provided which operate in accordance with different physical principles. For example, a photoelectric sensor can be provided for detecting thin paper sheets in transmitted-light operation and, additionally, a capacitive or inductive sensor may be provided for detecting metal-coated sheets.
In printing machines, the position of sheets on the front lays is decisive for the print quality that can be achieved. In the East German Patent Document DD 200563 A1, an individual sensor responsive to the presence of a sheet is provided for the purpose of detecting and evaluating a faulty feed of folded sheets in the conveying path. In addition, rows of a large number of individual sensors are provided which are arranged parallel and perpendicular to the conveying direction. With the aid of a microcomputer, the number of sensors which are covered by the sheet in the conveying direction and transversely thereto can be determined. The sheet format and a misaligned attitude of a sheet can be calculated from the number of the sensors and from the known distances between the sensors. If the dimensions and the angular attitude of a sheet exceed predefined limiting values, an error signal is generated. The resolution of the positional measurement in the conveying direction is limited by the smallest distance between the rows of sensors. The error signal can be used to activate a device for correcting the attitude and/or to stop the processing machine or the sheet singling or separating and the sheet feed.
For the aforementioned reasons, it has become known heretofore to provide ultrasonic detectors in the conveying path in order to monitor misfed and/or multiple sheets (note the published East German Patent Document DD 238 955 A1). Using a reference sheet, the ultrasonic detector arrangement, which includes a transmitter and a receiver, is calibrated. The receiver contains a comparator circuit having an adjustable threshold. During the feeding of sheets, the comparator flips into a second state thereof when an excessively thick sheet, or two or more sheets simultaneously run past the sensing location of the detector circuit. Due to the high gain of the comparator, the ultrasonic detector arrangement exhibits a quasi-digital behavior. With the aid of a differential amplifier, it is possible to reduce errors resulting from interfering variables and drift in the transmitter frequency and phase.
A device for determining double sheets disclosed in the German Patent 12 00 842 is based upon the detection of the amount of energy loss of longitudinal air-pressure oscillations as they pass through one or more sheets. In order to detect a double sheet, use is made of the fact that the energy loss is considerably greater when two sheets lie above one another than when the oscillations pass through only one sheet. This energy loss essentially arises at the interfaces between two sheets. The thickness of the sheet material and the coating thereof with printing ink or powder has only an insignificant influence. If the energy at the receiver for longitudinal air-pressure oscillations falls below an adjustable value, a switching operation is triggered via an amplifier. The device is constructed only for monitoring double sheets. A simultaneous detection of the attitude of the sheets in a sheet transport device is not contemplated here.
If two or more ultrasonic edge detectors are provided and are disposed transversely to the conveying direction, it is possible to detect a misaligned attitude of a sheet by is determining the time difference occurring at a leading edge of the sheet. Such an improvement is described in the published Japanese Patent Document JP 61-206758 (A).
In such attitude measurements which are based upon a time measurement, the accuracy of the attitude measurement depends to a considerable extent upon the stability of the trigger threshold of comparators. The time measurements are complicated and costly.